1. Field of the Invention
The present invention relates to a backlight driving technology, and particularly to an LED driving device and an LED driving method thereof.
2. Description of Related Art
For a flat panel display, it includes a liquid crystal displays (LCD), a field emission displays (FED), an organic light emitting diode (OLED), and a plasma display panel (PDP) in this field. Wherein, the LCD is widely adopted and has become a main stream of displays on the market due to its advantages of low operation voltage, radiation free, and so forth.
Generally, the LCD includes a liquid crystal display panel (LCD panel) and a light emitting diode (LED) driving device. LCDs are classified into a transmissive LCD and a reflective LCD. In the transmissive LCD, the back light source is provided by LEDs in back of the LCD panel, and the frames are viewed at the other of the LCD panel. Specifically, since the LCD panel does not emit light, the LEDs driven by the LED driving device are disposed in back of the LCD panel to provide the back light source required by the transmissive LCD. By providing the back light source, the back light passing through the LCD panel is polarized, so that colors displayed in the LCD panel are sensed by human eyes.
This kind of the LCD is usually applied to a display requiring high brightness, such as a desktop display, a personal digital assistant (PDA), and a mobile phone. However, with the development of LCD panel having large size, power consumption of the LED driving device used to the display requiring high brightness is more and more, and when the LED driving device is driven by high voltage, safety issue occurs due to voltage stress of the LED driving device. In order to better describe the issue due to the LED driving device, it is described with reference to a related figure in following.
FIG. 1 shows a conventional LED driving device. Referring to FIG. 1, the LED driving device 100 includes a voltage generator 110, a resistor 120, and an LED series 130, wherein the LED series 130 includes a plurality of LEDs coupled in series. When the LED driving device 100 is operated, the voltage generator 110 receives an input voltage, converts the input voltage to a DC voltage Vx, and outputs the DC voltage Vx from an end V11, so that the DC voltage Vx outputted from the end V11 is received by the LED series 130. The LED series 130 has an operating voltage Veff1, and the operating voltage Veff1 is a voltage difference between the end V11 and an end V12. The LED series 130 is coupled to one end of the resistor 120 through the end V12, and the other end of the resistor 120 is electrically connected to a ground GND.
Referring to FIG. 1, the voltage generator 110, the resistor 120, the LED series 130, and the ground GND form a current path. Accordingly, when the size of the LCD panel increases, the number of the LEDs in the LED series 130 increases with the size of the LCD panel, so that the operating voltage Veff1 of the LED series 130 also increases. In other words, relatively high voltage stress for the voltage generator 110 is generated, and safety issue occurs due to the voltage stress.
FIG. 2 shows another LED driving device for saving the issue of the voltage stress. Referring to FIG. 2, the LED driving device 200 includes a voltage generator 210, a resistor 220, and an LED parallel set 230, wherein the LED parallel set 230 includes a plurality of LED series 231 and 232 coupled in parallel. Wherein, when the LED driving device 200 is operated, the voltage generator 210 receives an input voltage and converts the input voltage to a DC voltage Vx, so that the DC voltage Vx outputted from the end V21 is received by the LED parallel set 230. The LED parallel set 230 has an operating voltage Veff2, and the operating voltage Veff2 is a voltage difference between the end V21 and an end V22. The LED parallel set 230 is coupled to one end of the resistor 220 through the end V12, and the other end of the resistor 220 is electrically connected to the ground GND. Referring to FIG. 1 and FIG. 2, for the LED series 130 and the LED parallel set 230, which include the same number of the LEDs, the numbers of the LEDs respectively included in the LED series 231 and 232 coupled in parallel in FIG. 2 are less than the number of the LEDs in the LED series 130 in FIG. 1. That is, the operating voltage Veff1 of the LED series 130 in FIG. 1 is larger than the operating voltage Veff2 of the LED parallel set 230 in FIG. 2. However, although the voltage stress of the LED parallel set 230 in FIG. 2 is reduced, and the safety issue is prevented, the equivalent resistors of the LED series 231 and 232 are mismatched, and further, the current passing through the LED series 231 and 232 is not equal, so that the brightness of the LED series 231 and 232 are not uniform.